1. Field of the Invention
This invention relates to a Langmuir probe for the measurement of plasma characteristics in plasma processing systems operating at radio frequencies (RF). In the present specification, RF means the frequency range from 0.1 to 1000 MHz.
2. Prior Art
Plasma processing is used in many industrial applications including semiconductor device, micro-machines, thin-film and nano-technology fabrication. These industrial plasmas are created by applying power to a typically rarefied gas mixture in a confined reactor. The plasma consists of ions, electrons, radical gas species and neutral gas, all of which permit the desired reaction to proceed. A substrate to be processed is located in the plasma reactor according to various configurations. Power is also applied in various configurations, depending on the particular process. A typical plasma process uses power applied at radio frequencies, the advantages being process efficiency and the ability to process dielectric substrates.
In the design of either the plasma reactor or the plasma process it is instructive to know key plasma characteristics such as electron density and temperature, ion density, electron energy distribution, plasma potential and floating potential. One device that has been used extensively to measure these plasma characteristics is the Langmuir probe.
Essentially, a Langmuir Probe consists of a short thin metal wire inserted into the bulk plasma. By applying a voltage to this wire and measuring the resultant drawn current, it is possible to construct a current-voltage (I-V) curve characteristic of the plasma. From the I-V characteristic, plasma parameters such as plasma floating potential, plasma potential, electron temperature, electron number density, electron energy distribution function and ion density are derived. The Langmuir probe can be moved in any direction in the plasma and used to build a map of these fundamental plasma characteristics. This information is of great benefit to scientists and engineers with an interest in characterising plasma processes or optimising the design of either the plasma reactor or the plasma process. Conventionally, Langmuir probes as described above have been used to measure plasma characteristics in direct current (DC) powered plasma.
However, since many industrial applications employ RF powered plasmas, techniques have evolved to modify the simple Langmuir probe. This is necessary since the simple Langmuir probe assumes a stationary plasma potential, whereas in the RF powered plasma the plasma potential varies at the frequency of the driving power. Using a simple Langmuir probe in an RF plasma results in a distorted I-V characteristic with associated errors in the inferred plasma characteristics. Typically, the plasma potential is less distinct, the floating potential and electron density are underestimated, and the electron temperature is overestimated. The shape of the I-V characteristic is clearly distorted.
Several techniques have been described which attempt to eliminate the error due to the RF interference. For example, one technique, described by Paranjpe et al. “A tuned Langmuir Probe for measurements in RF glow discharges”, in J. Appl. Phys. Vol. 67, pp. 6718, 1990, reduces RF distortion by increasing the impedance of the Langmuir probe at the RF driving frequency. This is done by including a series tuned filter in the probe circuit so that the probe presents a large impedance for the plasma driving frequency. This large impedance between the probe tip and ground ensures that the distorting RF voltage appears between the probe and the reference ground potential and not between the probe tip and the plasma. Minimising the RF distorting voltage between the plasma and the probe tip is key to accurate measurement of the plasma characteristics. For example, to measure an electron temperature of 1 eV, typical of some low pressure RF plasmas, the RF distortion voltage appearing between the probe trip and the plasma should be less than 1V. The RF plasma can vary by hundreds of volts during the RF driving frequency cycle. Therefore the tuned filter must have very high impedance to ensure the RF voltage between the probe tip and plasma is minimised.
Commercially available Langmuir probes for RF plasmas tend to use the tuned filter method, fixing the optimum filter response frequency for the application of interest. FIG. 1 is a schematic cross-section through a typical commercial Langmuir probe for RF plasmas. It comprises an elongated metal conductor 10 having a tip 1 which in use is in direct contact with the plasma, and a rear or outer end 3. The conductor 10 has a tuned filter 4 in series between the tip 1 and the outer end 3. The conductor 10 is surrounded by a hollow tube 2, usually constructed from a dielectric material, which also houses the tuned filter 4. The tube 2 is vacuum sealed at its outer end to the wall 7 of the plasma chamber, and itself contains a vacuum seal 5 to isolate the high vacuum plasma chamber from the ambient atmosphere. Building the tuned filter 4 within the tube 2 avoids capacitive loading to ground via the chamber wall 7. Some commercial RF Langmuir probes use a further extension of the tuned system to decrease the RF distortion effect. This is an RF compensation electrode 6 in contact with the plasma and capacitively coupled to the tip 1. Usually, the compensation electrode is a metallic cylinder which surrounds the inner end of the tube 2. The capacitance is designed to be large enough to effectively shunt the plasma-to-probe tip impedance. Hence a low impedance path from the probe tip to the plasma allows the probe to float at the oscillating plasma potential, further reducing the effect of the RF distortion. In use the outer end 3 of the conductor 10 is connected to external measuring circuitry by which the probe tip 1 is biased through a range of voltages and the resulting current collected to generate the I-V characteristic.
FIG. 2 shows an equivalent circuit model of the prior art RF Langmuir probe and the plasma-probe interface (the plasma-probe interface is the electron-depleted region which surrounds the probe tip 1 in use and separates the tip from the bulk of the plasma). As described above a large RF potential 21 appears at the plasma-probe interface. This interface is represented by capacitance 23 and parallel resistance 24. The existence of the RF potential across this interface causes the RF distortion of the probe's I-V characteristic, generated by varying a DC voltage 22 and measuring the resultant current. The tuned probe and compensation electrode method attempt to reduce the RF distortion by adding a shunt capacitance 26 (the compensation electrode) and a filter 27, consisting of an inductance 28 and parallel tuning capacitance 29, resonant at the RF driving frequency (the compensation electrode capacitance is shown as a single capacitance for simplicity, whereas in reality it consists of the probe-to-electrode capacitance in series with the electrode-to plasma-capacitance). A parasitic capacitance 25 to ground 30 adds to the distortion if the tuned circuit is outside the chamber wall. If the tuned circuit is built within the chamber wall, then the effect of the parasitic capacitance is much less.
Essentially, then, state of art RF Langmuir probes attempt to reduce the effect of the RF distortion by minimising the RF voltage appearing between the plasma and the probe tip. Firstly, the probe-to-ground impedance at the RF driving frequency is increased using the tuned filter. This tuned filter is typically highly resonant at the plasma driving frequency, thereby reducing RF distortion as much as possible. Secondly, the plasma-to-probe impedance is reduced by introducing a capacitive shunt impedance.
A modification of the tuned probe technique is described in U.S. Pat. No. 5,339,039. This system uses a semiconductor FET and parallel capacitor to form the tuned circuit, ensuring optimum tuning for the particular plasma driving frequency.
One disadvantage of the tuned probe systems as described is that the system is tuned to operate at a particular frequency, namely the RF plasma driving frequency. The system must be modified to operate in a plasma using a different driving frequency. Also, many RF plasma now utilise dual frequency RF power, so that the probe as described cannot be used, since the resonance point of the filter is tuned only to a single frequency.
Another disadvantage is the difficulty in designing and optimising the tuned filter. In one commercial system, the tuned filter is a self-resonant inductor, i.e. the inductor has a resonance with the self-capacitance of the inductor windings close to a desired frequency. One known disadvantage of this system is that the inductor has a low current rating thereby limiting the operating plasma density.
There is a need therefore for a Langmuir probe system capable of measuring plasma characteristics at a range of RF driving frequencies and also in plasma systems driven by more than one frequency.